kemenaran / pico8-boot

A reproduction of the pico8 boot animation on Game Boy Color

This code reproduces the pico8 boot animation on a Game boy Color.

Implementation

The boot animation is composed of 6 frames.

Each frames changes the color of many pixels – but the palette itself never changes (it is a fixed set of 32 colors).

To achieve the effect:

• On each line (hblank), the BG tile attributes are DMA-ed to reference the proper colors. There's 16 bytes to transfer. This allows to display 4 colors per 8x1 line (instead of 4 colors per 8x8 block).
• On each frame (vblank), the BG tile data is updated with the new pixels.

Issues

• It takes a while to load the tileset and tilemap. If needed, we can HDMA them during HBlank (one tile per scanline)

• On the original boot animation, somes 8x1 lines feature 5 differents colors (4 colors + black backround). The GBC is limited to 4 colors per 8x1 line

• Although we can change the palette or tile attributes on each scanline, each 8x1 line would need to pick one of the 8 available palettes. So all 16 pixels blocks on a line would need to share 8 palettes.

Hardware infos:

• We can change up to 23 colors per scanline (maybe more if we skip black)
• 8 palettes of 3 colors + black is 24 colors.
• So we can change almost all colors during H-blank

Clues :

• For more colors per 8x1 block: add sprites to work around this
• Use the GBC screen remanence to display more colors
• Change the Y-scroll during scanline rendering, for more color (https://github.com/LIJI32/GBVideoPlayer/blob/master/How%20It%20Works.md)

Transforming the original image:

1. Copy the first column of pixels to the last, to restore symetry ;
2. Translate the image by 1px, for tiles to be aligned to a 8x8 grid ; both reduce cases of 5 colors per 8x1 to 4
3. Crop half of the image, and duplicate it on render (reduce number of palettes per scanline from max 16 to max 8)
4. TODO: Rotate the original artwork by 1px every line, and back-correct by changing the X-scroll of 1px during HBLANK (see if it helps to reduce the line-by-line diff)

Tools

Palettes converter:

• https://orangeglo.github.io/BGR555/ (use Big Endian)

Images conversion:

• convert 1.small.png +dither -colorspace Gray -colors 4 -depth 4 1.bw.png
• rgbgfx original-gfx/4-colors/1.bw.png --unique-tiles --tilemap gfx/1.bw.tilemap --output gfx/1.bw.tileset.2bpp
• rgbgfx gfx/3.bw.tileset.png --reverse 16 --output gfx/3.bw.tileset.2bpp

Changing palettes mid-scanline demos: https://github.com/EmmaEwert/gameboy https://noncircadian.tumblr.com/post/162989007715/2-bits-per-pixel

TODO

• ✅ Display the first frame in DMG mode
• ✅ Implement double-buffering
• ✅ Switch to DMG-on-GBC (but with double speed)
• ✅ Why aren't the second frame tiles written to VRAM bank 1? (Because we're not in CGB compatible mode!)
• ✅ Convert the ROM to GBC
• ✅ Split loading into several stages (it seems 224 tiles can be loaded per vblank max)
• ✅ DMA tiles and tilemap
  • ✅ Align tilesets to $10 boundaries
  • ✅ Optimize tilemap load to fit a VBlank period
  • ✅ DMA only the allowed size (split large tilesets in two)
  • ✅ Rewrite tileset datastructures as structs (instead of several arrays)
• ✅ First frame is black
• ✅ Diplay all frames in non-colored mode
• ✅ Add basic colors
• ✅ Fix order of data loading and buffer swap (to avoid a flash of colors)
• Color-correct the pico8 colors
• ✅ Write a script to output color statistics about an image: color count per picture/per tile/per 8x2 bloc
• ✅ Write a script to rotate the image by 1px every line, and see if the stats are better
• Code a demo to see how many colors can be updated every scanline
• Write a script to output a new image with reduced colors and/or tiles that can be displayed by the GBA